Processed meat products, such as sausage, are often packaged in casings made from thermoplastic, heat-shrinkable films. Although some casings have a lay flat width of 6-20 inches, some products, such as liver sausage, etc., are quite often packaged in casings of smaller width, e.g., widths of 3-6 inches. Such casings often may need to have a precisely-controlled width, because the packaged products are sliced at uniform intervals. The slices need to be of uniform weight, because the packages are sold with a stated number of slices, and the package is stated as having a specific weight. If the diameter of the casing is not uniform, the weight of the slices will vary to an unacceptable degree.
Thus, there is a need for casings having a small and uniform diameter. In order to achieve this result, backseamed casings have been provided, these casings having a longitudinal seal, i.e., backseam, which enables a precisely controlled casing width, independent of film extrusion variations. In the production of backseamed casings, e.g., using a backseaming machine such as a Nishibe HSP-250-SA backseaming machine obtained from Nishibe Kikai Co. Ltd. of Nagoya, Japan, a flat sheet of film is folded longitudinally by passage over a "forming shoe." (A forming shoe is part of the backseaming machine which the film is passed under and around, i.e., so that the initially flat film is reconfigured as a tube, having a longitudinal overlap and seal therealong, with the width of the tube being determined by the circumference of the forming shoe.) A longitudinal lap or butt seal is then applied while the film is between the forming shoe and a sealing device, so that a seamed casing is formed. Butt seal casings have a butt seal tape sealed to one of the outer surfaces of the casing film. In either event, the resulting tubing, termed a "backseamed casing," is sealed or clipped at its ends after being filled with a meat product. For some uses, the meat product is thereafter cooked while in the backseamed casing.
It is also desirable to provide a film suitable for cook-in end use, which film adheres to meat products having relatively low protein content, i.e., high-fat/low-protein meat products. Of course, it is desirable to provide a backseamed casing in which the backseam seal survives the cook-in process, and in which the backseamed casing is of highly uniform width.
Typically, a polar surface is needed for adhesion of the film to meat products having relatively low protein content. This polarity can be provided either by using polar resins or through surface modification such as corona treatment. Typically, polar polymers used for meat adhesion include polymers such as ethylene/unsaturated acid copolymer, anhydride-containing polyolefin, and polyamide. However, ethylene/unsaturated acid copolymer and anhydride-containing polyolefin often do not provide adequate meat adhesion to these low protein containing meat products, whereas polyamide can be used to provide adequate meat adhesion for low protein/high fat meat products such as liver sausage, bologna, mortadella, etc.
Furthermore, adhesion of meat to a film can be enhanced by corona treating the surface of the film to which the meat is to be adhered. However, inferior seals, i.e., leaky seals, can result upon sealing a film which is corona treated, relative to the same film which is not corona treated. This leaky seal problem characteristic of such corona treated films can be avoided by "buffing off" the corona treatment in the area of the seal, so that the advantageous effects of the corona treatment, i.e., greater meat adhesion, especially to high fat meat products, can be retained, while at the same time avoiding the seal problems caused by the corona treatment. However, the buffing step is undesirable, as it is an additional processing step, and is frequently inconsistent.
Furthermore, if backseaming is carried out after corona treatment, the shrinkage of the film against the forming shoe rubs the film against the forming shoe edges, thereby reducing or destroying the increased meat adhesion from the corona treatment, at least in the area in which the film rubbed against the forming shoe. As a result, such films exhibit purge or fatting out at those locations, an undesirable effect. Furthermore, corona treatment has been found to be inconsistent, at least with respect to prevention of purge for products relatively low in protein content. As a result, it would be desirable to provide a corona-treatment-free backseamed casing which prevents purge from products relatively low in protein.
Finally, oriented films containing an outer polyamide layer can have watermarks on the polyamide surface of the film if water is used as a heat transfer medium immediately prior to the orientation step. Watermarks can adversely affect subsequent printing, due to the roughness caused by the watermarks on the surface of the film. Although there are ways to prevent watermarks on the polyamide layer, such prevention means may be undesirable in that they complicate the process, and frequently do not entirely eliminate the watermarks. Accordingly, it would be desirable to eliminate watermarks entirely, or at least eliminate the detrimental effects of watermarks.